Electropolishing of zinc-copper alloys



United States Patent 01 3,088,887 ELECTROPOLISHING F ZINC-COPPER ALLOYS Joseph Adrien M. Le Duc, Painesville, Ohio, assignor to Diamond Alkali Company, Cleveland, Ohio, :1 corporation ofDelaware No Dr hwing. Filed Feb. 9, 1059, Ser. No. 791,840 9 Claims. (Cl. 204140.5)

This invention relates to the electropolishing of zinc and zinc alloys, particularly zinc alloys containing copper, and, more particularly, to the process for the anodic polishing of zinc and zinc alloys and to an electrolyte composition for use in such a process.

Zinc and Zinc alloys, particularly zinc alloys containing 90% or more zinc, are presently widely used as the base material for making plated articles, particularly chromium plated articles. In such uses, a casting is made of the zinc or zinc alloy, which casting is customarily then trimmed, strapped and buffed, generally in an automatic butfing machine, to smooth the surface and to eliminate, as much as possible, any pits therein. The surface of the casting is then degreased and cleaned, after which it is plated, first with copper, then nickel and finally chromium to give the finished plated article.

In this procedure, one operation, that of buffing, is done by hand and is the most time consuming and expensive, often accounting for as much as 50% of the total finishing cost. Additionally, because all metals possess some degree of malleability, mechanical polishing causes deforming and crushing of the crystal structure of the metal near its surface. This deformation and crushing adversely affects the friction and wear properties of the metal, as well as reducing the corrosion resistance and endurance strength of the metal even after plating.

In order to overcome the structural disadvantages which result from mechanical buffing, as well as to reduce substantially the time and cost of such an operation and obtain a deburrin g as well as brightening of the workpiece, it has been proposed to polish zinc and zinc alloy castings electrolytically. Where the casting to be polished is of zinc or a zinc alloy containing less than about 0.10% copper, excellent results have been obtained using an electrolyte comprised essentially of an alkali metal hydroxide. However, where the casting is a zinc alloy which contains more than about 0.10% copper, e.g., 1.0% or more, it has been found that such an electrolyte is not efiective in electrolytically polishing the casting.

In using an alkali metal hydroxide electrolyte with such castings, a black, copper oxide smut is formed on the surface of the casting, and there is no polishing or brightening thereof. It is believed that this black smut is formed because the alkali metal hydroxide, while removing zinc from the surface of the casting, does not remove the copper, but rather forms copper oxide which remains on the surface of the casting.

In addition to the difficulty encountered in electropolishing copper-containing zinc alloys using an alkali metal hydroxide electrolyte, it has also been found that difficulties are encountered even when electropolishing zinc and zinc alloy castings which do not contain copper in an alkali metal hydroxide electrolyte. The primary cause for this difficulty is that considerable quantities of fill 3,088,887 Patented May 7, 1963 spongy zinc are deposited on the cathodes of the electropolishing apparatus during its use. This spongy deposit of zinc on the cathode becomes larger as the electropolishing is continued, loosens itself from the cathode, floats on the electrolyte and becomes mixed therewith. This floating, spongy mass comes in contact with the anode workpiece, thereby short-circuiting above the solution, in a hydrogen-oxygen atmosphere, often resulting in an explosion.

It has now been found in the practice of the present invention, that by incorporating an alkali metal cyanide and an alkali metal thiocyanate in the alkali metal hydroxide electrolyte, zinc or zinc alloys regardless of their copper content, can be anodically electropolished. Additionally, it has been found, that where a cathode having a surface containing Fe 0 is used with the above electrolyte, there is no formation of spongy zinc on the cathode, the zinc remaining in solution in the electrolyte. Alternatively, an oxidizing agent or compound having available oxygen can be added to the electrolyte to prevent the formation of spongy zinc on the cathode.

It is, therefore, an object of the present invention to provide a commercially feasible process for the anodic electropolishing of zinc and zinc alloys, regardless of the copper content thereof.

A further object of this invention is to provide a suit able electrolyte for use in such a process.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

As used in the description of the invention in the claims, the terms alkali metal hydroxide, alkali metal cyanide and alkali metal thiocyanate are meant to refer to the hydroxides, cyanides, and thiocyanates of lithium, sodium, potassium, cesium, and rubidium. Also, because of similar properties in the practice of the present invention, barium can be substituted Where the term alkali metal otherwise appears in the description of the invention. However, because of their low cost and ready availability, the sodium compounds are preferred, and for this reason, primary reference will be made hereinafter to sodium hydroxide, sodium cyanide, and sodium thiocyanate.

In a commercially feasible process for anodically electropolishing zinc and zinc alloys, the method of the present invention envisions the following operative conditions. The electrolyte should contain an alkali metal hydroxide, or barium hydroxide, preferably sodium hydroxide, within the range of about 450 to 900 g./liter with the preferred range being 572 to 763 g./liter, i.e., about a 40 to 50% aqueous solution of sodium hydroxide. Additionally, the electrolyte should contain an alkali metal cyanide, or barium cyanide, preferably sodium cyanide in the amount of l to 100 g./liter, 10 to 15 g./liter being preferred and an alkali metal thiocyanate, or barium cyanate. preferably sodium thiocyanate in an amount of l to 200 g./liter, preferably 50 to 100 g./liter.

The temperature of the electrolyte during the electropolishing may vary within the range of to 200 F., depending upon the concentration of the alkali metal hydroxide in the electrolyte. At higher alkali metal hydroxide concentrations, e.g., 800 to 900 g./liter, the preassess? ferred temperature range is 125 to 162 F. However, with the preferred alkali metal or barium concentration of 572 to 763 g./liter, these high temperatures will not produce the most effective electropolishing of the zinc and zinc alloys, while temperatures within the range of 70 to 90 F. give the best electropolishing and hence are the preferred temperatures for this alkali metal hydroxide concentration range.

Within the above ranges of electrolyte compositions, at the preferred temperatures given, satisfactory electropolishing of zinc and zinc alloys is carried out over a wide range of anode current densities, e.g., 50 to 800 amps/square foot for various periods of time. For example, at the higher current densities, the time required for good electropolishing will be about 1 to 2 minutes, while at lower current densities, the time required will increase to 4 minutes or somewhat more. However, it will be appreciated by those skilled in the art, because of the heavier electrical load carrying equipment which is required to provide high current densities, which equipment is not standard in most electrolytic apparatus, anode current densities of about 200 amps/square foot are preferred. At this anode current density about 4 minutes is required for good electropolishing. Additionally, it

has been found that in the electropolishing process carried out under these conditions, the solution current density, which may be defined as the total applied current divided by the total effective volume of electrolyte, should not be in excess of about 2.5 amps/liter. Where this value is exceeded, the zinc or zinc alloy castings are polished, but zinc is plated on the cathode.

It has been found that even when operating the electropolishing process with the above electrolyte and under the above preferred conditions, a spongy, porous zinc is deposited on the cathode, which zinc deposit, if deposition is permitted to continue, will eventually cause a short circuit in the cell. It has now been found that this zinc deposit is eliminated by using a cathode having a surface which contains Fe O It is to be recognized that Fe O is frequently expressed as Fe O .FeO and such designation is included in the present invention. Examples of such material are heat treated steel, black oxide coated steel, hot-rolled steel, magnetite and any other metal or non-metal on which there is a surface which contains Fe O Additionally, by a cathode having a surface which contains Fe O it is intended to include cast iron which has likewise been found to exhibit the same desirable qualities as the materials mentioned above. Preferred among such cathodes is one of hot-rolled steel or black oxide coated steel, Such a cathode may be in the form of one or several plates or if desired may be the tank which contains the electrolyte. When such a cathode is used, there is no deposition of spongy, porous zinc thereon, the zinc remaining in solution as zincate with considerable quantities of hydrogen discharging at the cathode. For the most effective operation, that is, in order to obtain the most complete dissolution of the zinc into the solution, the cathode area should be in the order of to 40 times the area of the anode.

Alternatively, to prevent a build-up of zinc on the cathode, an oxidizing agent may be added to the electrolyte. Examples of such materials are perborates, peroxydisulfates, oxychlorides, permanganates, bichrornates, chlorates, bromates, iodates, nitrates, peroxides, and chlorine. Particularly good results have been obtained using the alkali metal perborates peroxydisulfates, peroxides and nitrates and the alkaline earth metal oxychlorides, specifically, sodium nitrate, sodium perborate, potassium peroxydisulfate, and sodium peroxide and calcium oxychloride. Although addition of these materials, in the amount of about to g./liter of electrolyte is effective in avoiding deposition of zinc at the cathode, these agents must be replenished about once every 8 hours. Because of this, the preferred method of preventing a build-up of zinc on the cathode is the use of a cathode having a surface which contains Fe O While the oxidizing agent in the electrolyte avoids deposition of zinc at the cathode, the exact mechanism by which the Fe 0 containing surface on the cathode prevents zinc deposition is not known. It is believed, however, that the hydrogen overvoltage value of the Fe O. coated cathode has greatly changed the characteristics of the system, so that the current does not deposit zinc on the cathode, but rather electrolyzes the contents of the electrolyte, hydrogen being given off at the cathode and the oxygen combining with the zinc and the alkali to form a zincate, which gradually builds-up in concentration during electropolishing.

Referring now to the electroyte, in addition to 572 to 763 g./liter sodium hydroxide, 10 to 15 g./liter sodium cyanide and 50 to 100 g./liter sodium thiocyanate, in the preferred electrolyte it has been found to be desirable to add a small amount, e.g., 0.003 to 0.03% by weight of material Which acts as a defoamer. Examples of materials which may be so used are the following:

Anisole Dimethyl octynediol Phenol Cocoyl surcosine Cresol Lauric isopropanolamide Mono-butyl naphthalene sodium sulfonate Di-butyl naphthalene sodium sulfonate Nonyl phenoxy polyoxyethylene ethanol Disodium-N-octadecylsulfosuccinamate Aliphatic substituted butynediols Aliphatic substituted octynediols Aliphatic substituted octynediols mixed with an alkyl phenyl ether of polyethylene glycol in ethylene glycol Alkyl benzyl poyethylene glycol ether Polyoxyethylene esters of mixed fatty and rosin acids Polyethylene tridecyl alcohol Polyethylene tridecyl alcohol and urea Branched chain alcohol ethers Alkyl phenyl polyethylene glycol ether Polyalkylene gycol ethers Polyoxyethyene ester Compounds formed by addition of propylene oxide to ethylene-diamine followed by addition of ethylene oxide Of these, cresol, aliphatic substituted octynediols, aliphatic substituted octynediols mixed with an alkyl phenyl ether of polyethylene glycol in ethylene glycol, alkyl polyethylene glycol ether and polyoxyethylene esters of mixed fatty and rosin acids are preferred.

Where the electrolyte solution is used without a defoaming agent, there is a considerable frothing and foam formation therein, which foam, by adhering to the anode and cathode, makes electropolishing of the workpieces extremely difficult. It will be appreciated that this foam may be removed by mechanical means or by use of overflow and recirculation tanks. This, however, is cumbersome and adds additional steps, as well as cost to the electropolishing operation, so that the inhibition of the foam by the addition of a defoaming agent is greatly to be preferred.

In actual operation, an open tank is made of hot-rolled steel or heat treated steel, which tank serves as the cathode. For every ampere of current to be applied, 0.665 liter of an electrolyte solution containing from 572 to 763 g./1iter of sodium hydroxide, 10 to 15 g./liter sodium cyanide and 50 to 100 g./liter sodium thiocyanate are placed in the cathode tank. This quantity of electrolyte gives a solution density not in excess of 2.5 amps/liter. The temperature of this solution is maintained within the range of to F. To the electrolyte is added one of the polishing is carried out for a period of about 4 minutes.

At the expiration of this time, the workpiece is removed from the electrolyte and is found to have a smooth, bright, lustrous surface, free of pitting and copper smut. Additionally, it is noted that the cathode tank is completely free of any deposit of spongy zinc, and there is little, if

any, foam or troth in the electrolyte, thus showing the effectiveness of the defoaming agent and of a cathode having a surface containing Fe O in preventing the deposition of zinc.

In order that those skilled in the art may better under- The work- 5 6 tion as specified in the 1957 ASTM Supplement, Part 2, page Aluminum 3.9 to 4.3%. Magnesium 0.03 to 0.06%. Copper 0.75 to 1.25%. Iron 0.075% maximum. Lead 0.005% maximum. Cadmium 0.004% maximum. Tin 0.002% maximum. Zinc Remainder.

During the electropolishing, the electrolyte is agitated by means of work bar agitation, the work bar having a 2 inch displacement/one second stroke. A source of DC. current is connected to the workpiece, and electropolishing is carried out for the desired length of time.

The examples given in the following table show varying electrolyte compositions, and the results obtained when using these compositions to electropolish the above designated Zamak-S alloy. In all of these examples, the electrolyte temperature is between 73 and 81 F., the voltage used is between 4 and 7 volts, and electropolishing is carried out for a period of 4 minutes.

TABLE I Composition, g./l. Anode Ex. Bath current Remarks No. density,

NaOH Other amps/It 1.--- 114 665 100, ZnO 200 Dull grey surface, no brightening, black copper smut.

400 88 665 100, ZnO, sat. NaCl 200 Do. 96 500 Sat. NaCl 200 D0. 101 665 50, sulfamic acid.-. 200 Do. 104 665 50, fiuoborlc ac1d. 200 Do. 116 665 15, Rochelle salt 200 Do. 400 Do. 117 665 15, Na citrate 200 D0. 400 Do. 665 100, Rochelle salt 288 go. o. 133 665 40, NarFeswNh Z38 Dull no brightening, dark brown film, no black copper smut.

0. 134 665 25, cyanuric chloride goo. 138 665 30, thloglycollio acid Dull, no brightening, grey film, no black copper smut. 140 665 100, NaIISO; 2188 Dull grey surface, no brightening, black copper smut.

s o. 141 665 50, sodium iormate 200 Do. 400 Do. 142 665 100, Nil- P107 200 D0 400 Do. 143 665 100, sodium hexametaphosphato Ilgo. o. 144 685 100, ethylenediamlne tetraacetic acld go. o. 121 665 10, NaCN 288 Surt a ce clean, semi-bright, no copper smut.

4 0. 127 665 100, NaSCN 200 Surtgce clean and smooth, semi-bright, no copper smut, light film,

400 o. 20-.. 123 665 10, NaCN; 50, NasCN Surface clean and smooth, bright, no copper smut.

drical cathode is about 1115 ml. A inch x 1 inch 70 workpiece, having a total surface area of about 0.01 square foot, is secured to a work bar which is lowered into the electrolyte. This workpiece is a zinc alloy known by the trade name Zamak-5 (trademark of the New Jersey Zinc Company) and having the following composi- 75 As shown in the above examples, an electrolyte consisting only of sodium hydroxide or sodium hydroxide and zinc oxide is not effective in polishing the Zamak-S alloy. Additionally, it is seen that the best electropolishing results are obtained when the electrolyte contains sodium hydroxide, sodium cyanide and sodium thiocyanate.

The examples in the following table show the results obtained when electropolishing a Zamak-S alloy using electrolyte containing varying amounts of sodium hydroxide, sodium cyanide and sodium thiocyanate. In these examples, the electrolyte temperature is between 75 and 81 F. and the voltage used between 4.5 and 9.3 volts, and the electropolishing is carried out for a period of 4 minutes.

TABLE II Composition, g., l. Anode Ex Beth current Remarks No. density,

NaOI-I Other amps/it.

21 99 600 160, NuCn 200 Dull grey surface, no brightening, no smut. 22... 118 665 30, NaSGN 200 Light brown film, bright underneath, no pitting or smut. 23. 122 665 25, NaCN SUIIiIDBG clean, semi-bright, no smut.

o. 24.-. 125 665 25, NEON; 50, Nos 200 D0. 25... 126 665 10, NaCN; 100, NEISCN 288 CleoB, semi-bright, no smut.

4i 0. 600 Do. 800 D0. 26--. 129 665 5, NaCN; 160, NaSCN 200 Do. 400 Do. 600 D0. 800 D0. 27--. 131 665 100, ZnO; 10, NoCN; 50, NaSCN 28g CleuIn), smooth, semi-brlght, no smut.

4 0. 600 D0. 800 Do. 28. 137 665 2, NaCN; 100, NuSGN 200 Clean and smooth, spots of brightness, no smut. 29. 139 763 15, NaCN; 60, NaSCN 2% Chang, smooth, semi-bright, no smut.

o. 600 Do.

In the following examples, electropolishing is carried out at varying anode current densities for a period of 4 amps/square foot, with the best results being obtained at about 200 amps/square foot.

In the following examples, electropolishing is carried minutes.

TABLE III Composition, g./l. Anode Ex. Bath current Volt- Temp, Remarks N 0. density, age F NaOH NaCN NaSCN amps/it;

30. 121A 665 10 50 3. 4 80 Dull, dark oxide surface, smooth, no smut. 100 4. 3 S Dull, light oxide surface, smooth, no smut. 200 5. 6 77 Clean, smooth, semi-bright. 400 6 4 78 D0. 600 7.3 75 Glenn, smooth, semi-bright, light film. 800 8. 5 7? D0.

31. 122A 665 25 5D 2. 8 80 Dull oxide surface, smooth, no smut.

100 3. 9 80 Clean, dull, smooth, no smut. 200 4. 7 77 Clean, semi-bright, smooth, no smut. 400 5. 4 78 Do. 600 8.2 75 Dull, dork oxide, no smut. 800 8. 4 77 D0.

32..... 123A 665 10 50 50 3.0 80 Do.

100 3. 7 78 Clean, semi-bright, no smut. 200 5. 8 77 Clean, semi-bright, no smut, smooth. 400 (i. 4 78 D0. 600 7. 2 75 Clean, light brown surface, dull, no smut. 800 8. 7 77 Do.

As seen in the above examples, electropolishing begins when using an anode current density between 50 and 100 out at three different current densities for varying lengths of time.

TABLE IV Composition, gJl. Anode Ex. Bath current Temp, Time, Remarks N0. density, F. min.

NaOH NaCN NaSCN amps/ft.

33. 122B 665 25 100 75 18 (310%, smooth, dull, no smut.

2 0. Do. 34. 122B 665 200 1g C1633, smooth, very little brightening, no smut.

o. 20 D0. 35. 122B 665 25 400 75 8 Dull and pitted, no smut.

12 Do. 20 Do. 36. 123B 665 16 100 8 Clean and dull, no smut.

12 Do. 20 Do. 37. 123B 665 10 50 200 83 18 (Elena, smooth and dull, no smut.

2 o. 20 Do. 38-. 123B 665 10 50 400 75 18 C1083, dull and partially pitted, no smut.

As seen in the above examples, polishing times of greater than 4 minutes do not give as good results as those obtained when electropolishing for only 4 minutes.

Example 39 A life test is made using liters of electrolyte containing 763 g./liter sodium hydroxide, g./liter sodium cyanide and 50 g./liter sodium thiocyanate, and 0.2 g./liter of polyoxyethylene ester of mixed fatty acid and rosin acid (Renex as a defoamer. The anode used are /2 inch strips of Zamak-S alloy having a total surface area of 0.034 square foot, while the cathode is 2 sheets, 7 inches x 7 inches of heat treated steel having a surface area of 0.58 square foot. The temperature of the electrolyte is maintained within the range of 69 to 90 F., the average temperature being about 80 F.

a temperature between about 60 and 200 F.; and passing an electric current through said electrolyte between said anode and a cathode having a surface consisting essentially of Fe O at an anode current density between about 50 and 800 amps. per square foot and a solution current density not in excess of about 2.5 amps. per liter, using a voltage sufficient to provide said current density but below that at which zinc will be electrodeposited on said cathode, thereby releasing hydrogen gas at said cathode while preventing zinc from plating on the same; and maintaining the passage of said electric current for a period of time sufficient to electropolish said article.

2. The method according to claim 1 wherein the cathode is made of hot-rolled steel.

3. A method for anodically electropolishing an article having a surface comprising a zinc alloy containing cop- TABLE V Anode Anode Bath Cumulative, current Cumulative, Free ZnO, Anode efli- N 0. hours and density, ampere \laOII, g./l. loss, ciency, Remarks minutes amps] hours g./l. grams perit cent;

3-21 200 Two small bright spots, balance dull. 325 400 About bright. 329 600 About 70% bright. 333 800 Almost 100% bright. G-l l 490 6l6 400 621 800 Bright. 12-38 400 1243 800 Bright, slight brown film. 12-48 800 Bright. 12-53 800 Do. 20-25 400 Bright, slight brown film. 2030 800 2ti-59 400 27- 4 800 27 9 S00 34-ld 400 3421 800 Increasing brown film and rough area. 34-25 800 Foam, high polish, rough edge. 3429 800 Bright. no brown film. 38-11 400 corroded. rough.

From the tables given above, it is seen that a zinc alloy containing appreciable quantities of copper can be electropolished using an electrolyte containing sodium hyroxide, sodium cyanide and sodium thiocyanate. It is further shown, that the best electropolishing results are obtained using an anode current density of about 200 amps] square foot, and an electropolishing time of about 4 minutes.

Although there have been described various embodiments of the invention, the methods described are not intended to be understood as limiting the scope of the invention, as it is realized that changes therewithin are possible, and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A method for anodically electropolishing an article having a surface comprising a zinc alloy containing cop per, which method comprises making the article to be electropolished the anode in an electrolytic cell, the electrolyte of which consists essentially of an aqueous solution of a compound selected from the group consisting of alkali metal hydroxides and barium hydroxide in an amount between about 450 and 900 g. per liter, a com pound selected from the group consisting of alkali metal cyanides and barium cyanide in an amount between about 1 and 100 g. per liter, and a compound selected from the group consisting of alkali metal thiocyanates and barium thiocyanate in an amount between about 1 and 200 g. per liter, said electrolyte being maintained at per, which method comprises making the object to be electropolished the anode in an electrolytic cell, the electrolyte of which consists essentially of an aqueous solution of an alkali metal hydroxide in an amount between 562 and 763 g. per liter, an alkali metal cyanide in an amount between 10 and 15 g. per liter and an alkali metal thiocyanate in an amount between 50 and 100 g. per liter, said electrolyte being maintained at a temperature betwen about 70 and F.; and passing an electric current through said electrolyte between said anode and a cathode having a surface consisting essentially of F3 0,, at an anode current density of about 200 amps. per square foot and a solution current density not in excess of about 2.5 amps. per liter, using a voltage sufficient to provide said current density but below that at which zinc will be electrodeposited on said cathode; and maintaining the passage of said electric current for a period of time suflicient to electropolish said article.

4. The method according to claim 3 wherein the cathode is made of hot-rolled steel.

5. The method according to claim 3 wherein the alkali metal is sodium.

6. The method according to claim 5 wherein a defoaming agent is added to the electrolyte.

7. A method for anodically electropolishing an article having a surface comprising a zinc alloy containing copper, which method comprises making the article to be electropolished the anode in an electrolytic cell, the electrolyte of which consists essentially of an aqueous solution of sodium cyanide in the amount of about 665 g. per liter, sodium cyanide in the amount of about 15 g. per liter and sodium thiocyanate in the amount of about 50 g. per liter, said electrolyte being maintained at a temperature of about 76 F.; and passing an electric current through said electrolyte between said anode and a cathode having a surface consisting essentially of Fe O at an anode current density of about 200 amps. per square foot and a solution current density not in excess of about 2.5 amps. per liter, using a voltage sufficient to provide said current density but below that at which zinc will be electrodeposited on said cathode; and maintaining the passage of said electric current for a period of time suflicient to electropolish said article.

8. The method according to claim 7 wherein the cathode is made of hot-rolled steel.

9. The method according to claim 7 wherein a defoaming agent is added to the electrolyte.

References Cited in the file of this patent UNITED STATES PATENTS 931,513 Specketer Aug. 17, 1909 2,101,580 Henricks Dec. 7, 1937 12 Oplinger Feb. 7, 1939 Oplinger Apr. 18, 1939 Ward Sept. 11, 1945 Jernstedt Oct. 12, 1948 Eaton Sept. 9, 1952 Hilliard Oct. 13, 1953 MacLachlin July 16, 1957 Fisher Mar. 28, 1958 FOREIGN PATENTS Great Britain July 12, 1949 Great Britain Feb. 23, 1955 OTHER REFERENCES Nature, vol. 142, 1938, pp. 477478 and 1161. Chemical Abstracts, 1942, vol. 36, p. 6919. 

1. A METHOD FOR ANODICALLY ELECTROPOLISHING AN ARTICLE HAVING A SURFACE COMPRISING A ZINC ALLOY CONTAINIG COPPER, WHICH METHOD COMPRISES MAKING THE ARTICLE TO BE ELECTROPOLISHED THE ANODE IN AN ELECTROLYTIC CELL, THE ELECTROLYTE OF WHICH CONSISTS ESSENTIALLY OF AN AQUEOUS SOLUTION OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL HYDROXIDES AND BARIUM HYDROXIDE IN AN AMOUNT BETWEEN ABOUT 450 AND 900 G. PER LITER, A COMPOUND SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL CYANIDES AND BARIUM CYANIDE IN AN AMOUNT BETWEEN ABOUT 1 AND 100 G. PER LITER, AND A COMPOUND SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL THIOCYANATES AND BARIUM THIOCYANATE IN AN AMOUNT BETWEEN ABOUT 1 AND 200 G. PER LITER, SAID ELECTROLYTE BEING MAINTAINED AT A TEMPERATURE BETWEEN ABOUT 60* AND 200*F.; AND PASSING AN ELECTRIC CURRENT THROUGH SAID ELECTROLYTE BETWEEN SAID ANODE AND A CATHODE HAVING A SURFACE CONSISTING ESSENTIALLY OF FE3O4, AT AN ANODE CURRENT DENSITY BETWEEN ABOUT 50 AND 800 AMPS. PER SQUARE FOOT AND A SOLUTION CURRENT DENSITY NOT IN EXCESS OF ABOUT 2.5 AMPS. PER LITER, USING A VOLTAGE SUFFICIENT TO PROVIDE SAID CURRENT DENSITY BUT BELOW THAT AT WHICH ZINC WILL BE ELECTRODEPOSITED ON SAID CATHODE, THEREBY RELEASING HYDROGEN GAS AT SAID CATHODE WHILE PREVENTING ZINC FROM PLATING ON THE SAME; AND MAINTAINING THE PASSAGE OF SAID ELECTRIC CURRENT FOR A PERIOD OF TIME SUFFICIENT TO ELECTROPOLISH SAID ARTICLE. 